cases – Numbered cases environment

De­fine the en­vi­ron­ment num­cases: equa­tions with sev­eral
al­ter­na­tive right-hand sides, with equa­tion num­bers for each
al­ter­na­tive. Also en­vi­ron­ment sub­num­cases, where each
al­ter­na­tive is a sub-num­ber (e.g., 8a, 8b, ...) of the equa­tion
set as a whole.